Online publications
Prof. Dr. Klaus Weltner
Survey:
- Misinterpretations of Bernoulli's
Law
- Physics of Flight - reviewed
Summaries:
Misinterpretations of Bernoulli's
Law
Abstract: Bernoulli's law
and experiments attributed to it are fascinating. Unfortunately some of
these experiments are explained erraneously, e.g.: the function of a vaporizer
and the soaring of a ping-pong ball in a jet stream of a hair dryer can
not be used as applications of Bernoulli's law. The static pressure in
a free jet stream is equal to the static pressure in the environmental
atmosphere regardless of the streaming velocity of the jet. This can be
shown by classroom experiments.
Acceleration of air is caused by pressure gradients. Air is accelerated
in direction of the velocity if the pressure goes down. Thus the decrease
of pressure is the cause of a higher velocity. It is wrong to say that
a lower pressure is caused by a higher velocity.
Pressure gradients perpendicular to the streamlines are caused by the deflection
of streaming air. The deflection of air generates regions of lower and
higher pressure according to the curvature of the streamlines. Vaporizer,
the soaring ping-pong ball as well as the physics of flight are only to
be explained regarding the acceleration perpendicular to the streamlines.
Physics of Flight - reviewed
Abstract: The conventional or standard explanation of aerodynamic
lift states the higher streaming velocity at the upper side of the airfoil
as cause of the lower pressure, due to Bernoulli's law. But a higher streaming
velocity is the effect of a lower pressure and never its cause. The cause
of the aerodynamic lifting force is the downward acceleration of air by
the airfoil - which depends on the angle of attack and its velocity.
In relation to the airfoil the normal acceleration of the air in case of
curved streamlines must be regarded which results in pressure gradients
perpendicular to the streamlines and reaction forces acting perpendicular
on the deflecting surfaces
Belarus Version by Galina Miclosic
For questions and comments, please send e-mail
to Prof. Weltner
Red.: Lewinsky
Didactic of Physics
20. Mai 1999